Insights into the unexpected formation of hexamethylbenzene during steam reforming of dimethyl ether over zeolite-based bifunctional catalysts

The effect of the acidity and topological structure of zeolites on the performance of the steam reforming of dimethyl ether (SRD) was comparatively investigated over the bifunctional catalyst composed of zeolite and Cu/ZnO/Al2O3 under the conditions of DME/H2O/N2 = 1/4/5, catalyst loading of 0.9 g, GHSV = 4000 mL g−1 h−1. The bifunctional catalyst was prepared by physically mixing Cu/ZnO/Al2O3 with H-ZSM-5 (Si/Al2O3 = 25), H-USY, and H-beta (Si/Al2O3 = 20), respectively. The acidic properties of the zeolite were characterized by NH3-TPD technique. Over all the bifunctional catalysts with a Cu/ZnO/Al2O3 to zeolite weight ratio of 1, SRD was dominantly occurred, and H-ZSM-5 based bifunctional catalyst showed the highest SRD performance, i.e., DME conversion of over 95% and H2 yield of about 80%. Moreover, light hydrocarbons were formed under SRD conditions as a result of DME (methanol) to hydrocarbons reactions (DTH, MTH). The results were well explained based on the two-step mechanism of SRD and the acidic properties of the zeolite. Unexpectedly, a white powdery product, which is determined to be hexamethyl benzene (HMB) by FT-IR, 1H and 13C NMR, GC/MS, and elemental analysis, was condensed at the outlet of the reactor when H-USY or H-beta based bifunctional catalysts were applied for SRD. Furthermore, the formation of HMB was more pronounced over the bifunctional catalyst by using H-beta as an acidic component. To the best of our knowledge, this is the first report on the formation of HMB under SRD conditions. Based on the results by changing the weight ratio of Cu/ZnO/Al2O3 to H-beta, the SRD mechanism, and the topological and acidic properties of H-beta, the formation of HMB was well explained as the side reactions of MTH and DTH and the larger channel sizes of zeolites Y and beta than that of the HMB molecules. Importantly, on the one hand, our findings can shed some lights on screening suitable solid acids for SRD. On the other hand, clear evidence is provided for the hydrocarbon pool mechanism of MTH reaction, and HMB is revealed to be the very possible species of hydrocarbon pool.

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► Effect of zeolite structure and acidity on SRD was comparatively investigated.
► The acidity and structure of different zeolites play a key role for SRD.
► H-ZSM-5 based bifunctional catalyst is active and selective for SRD.
► First report on the formation of hexamethylbenzene under SRD conditions.
► Hexamethylbenzene is a possible hydrocarbon pool species for MTH reaction.